The applicants have already advocated a method for the continuous heat treatment of thin steel sheet comprising a rapid cooling stage consisting of immersion in an aqueous bath at a temperature higher than 75.degree. C., followed by an overaging stage (e.g. British patent specification No. 1,497,502). This method is advantageous in that it ensures good homogeneity of the properties of the steel and enables the manufacture of sheet for drawing and medium strength sheet. In addition, the heat energy contained in the sheet is converted into steam energy during the immersion of the sheet in the cooling bath, which steam may be readily recycled or the latent heat of the steam may be recovered.
British patent specification No. 1,514,270 and corresponding Belgian Patent Specification No. 835,866 proposes a further rapid cooling method which enables the production of the same grades of steel, but without the possible recovery of energy and without the possibility of discontinuing the rapid cooling at the overaging temperature. However, this method enables the production of steel having a very high strength by very rapid cooling to ambient temperature, this method being of a very low cost in terms of alloying elements.